AccountsController Implementation For Register API

AccountsController.cs

using Authentication.Apis.Models;
using Microsoft.AspNetCore.Identity;
using Microsoft.AspNetCore.Mvc;
using System;
using System.Linq;
using System.Threading.Tasks;

namespace Authentication.Apis.Controllers
{
    [Route("[controller]/[action]")]
    [ApiController]
    public class AccountsController : Controller
    {
        private readonly UserManager<AppUser> _userManager;

        public AccountsController(UserManager<AppUser> userManager)
        {
            _userManager = userManager;
        }

        [HttpPost]
        public async Task<IActionResult> Register([FromBody] RegistrationInputModel registrationInput)
        {
            var user = await _userManager.FindByNameAsync(registrationInput.UserName);
            if (user != null)
            {
                return BadRequest("User already exists.");
            }

            user = new AppUser
            {
                Id = Guid.NewGuid().ToString(),
                UserName = registrationInput.UserName,
                Email = registrationInput.Email
            };

            var result = await _userManager.CreateAsync(user, registrationInput.Password);
            if (!result.Succeeded)
            {
                var resultErrors = result.Errors.Select(e => e.Description);
                return BadRequest(string.Join("\n", resultErrors));
            }

            return Ok(user);
        }
    }
}